1. Field of the Invention
This invention relates to a circuit for use in providing an improved degree of linearity between a meter input and a meter display and more particularly to such a circuit which compensates for specific un-predictable non-linearities which differ from meter to meter.
2. Description of the Prior Art
Analog meter movements have several kinds of basic non-linearity sources which limit meter accuracy. Such meters usually have a driven indicator which deflects from 90.degree. to 120.degree. from zero input to full scale input. Meters of this type are usually manufactured so that the rated full scale deflection angle is obtained by an absolute current which is variable by about plus or minus 5%. This presents no problem since the meter drive is adjustable in most present day meter applications. Since the meter drive current may be varied to obtain full scale meter deflection most plots of meter linearity are based on percent of full scale current rather than on percent of absolute current because, within some narrow range, it is not important what the absolute current is. The meter manufacture also provides a mechanical adjustment to position the meter indicator at zero with zero drive current into the meter input. As a consequence a mechanical adjustment may be made to adjust the zero position of an analog meter and an electrical adjustment may be made to adjust the meter input current to provide for full scale meter indicator deflection when full scale current is being measured.
Another basic source of analog meter movement non-linearity is the magnetic circuit in the common construction of such meters which dictates that the deflection angle of the meter indicator and the current that flows into the meter do not posses a completely linear relationship. However, the non-linearity arising from the current-magnetic circuit relationship is predictable and compensation for such non-linearity may be made by adjusting the plot of the meter scale. There are additional non-linearities which remain which are variable from meter to meter within a given production run of meters. Compensation in advance for these variable non-linearities are not practicable. As a consequence, meters which must operate within a specification recited as two or one percent accuracy require built in adjustments at the manufacturer's facility. A degree of success has been seen by selectively demagnetizing the magnet in the meter movement. In such a case if a two percent accuracy meter is obtained and operated at one quarter scale and if the maximum specified error exists at one quarter scale then the error becomes eight percent of the one quarter scale reading since the two percent specification is in terms of full scale reading. It may therefore be seen that it is relatively difficult to obtain a meter which can indicate values within two percent of the indicated value down into the lower portions of the meter scale.
One meter linearizing scheme is known which uses a circuit containing diodes to provide different slopes in various sections of the meter curve. However, such a scheme causes the upscale range from that portion of the range where the slope is being adjusted to be effected by an adjustment in the lower part of the range. This interaction of the adjustment in an error compensation circuit for use with a meter makes the adjustment over the entire scale difficult and thereby makes the end product meter very expensive.
What has gone before describes the situation where, presuming an ideal input or drive signal, the linearity of a display device, whether it be digital or analog, is less than that desired and where means for correcting the departure from the desired display device character are either only marginally successful or so complex that they are prohibitively expensive. The situation also exists where the character of the input signal, presuming an ideal display device, departs from some desired character whether it be linear or some predetermined non linearity. In such a case current methods and apparatus for providing correcting signals to obtain the desired characteristics in the input signal suffer from the same drawbacks as exist in the case of display device deviations. It may be seen that the two problem areas are closely related and that some means is desirable for obtaining fine adjustments in each case as a supplement to those operations now performed which obtain displayed results in the aforementioned 2% of full scale region.